1. Field of the Invention
The present invention relates to a pneumatic tire which can discharge static electricity generated in a vehicle body and a tire to a road surface.
2. Description of the Related Art
Recently, there have been proposed a pneumatic tire including a tread rubber blended with silica at a high ratio in order to reduce the rolling resistance that largely affects the fuel consumption of a vehicle and/or to increase braking performance (WET braking performance) on a wet road surface. However, compared to a tread rubber blended with carbon black at a high ratio, the electric resistance of such tread rubber is high, and accordingly, static charge generated on a vehicle body or the tire is prevented from being released to the road surface. As a result, problems like radio noises tend to occur.
Accordingly, there has been developed a pneumatic tire which can achieve electrical conduction performance by forming a conductive portion which is made of a conductive rubber blended with carbon black or the like, in a tread rubber which is made of a non-conductive rubber blended with silica or the like. For example, in a tire described in Patent Documents 1 to 3, as shown in FIGS. 5 to 8, tread rubbers 10a to 10d having a cap portion made of a non-conductive rubber are provided with conductive portions 18 to 21 dividing the tread rubbers in a tire width direction, thereby forming a conductive path for discharging static electricity.
Meanwhile, according to a study of the present inventor, it is found that in the tread rubber 10a in FIG. 5, a support from a lateral direction by the conductive portion 18 is not sufficient with respect to the non-conductive rubber at the time of cornering, rigidity between an outer side and an inner side of the tire is off balance, and steering stability tends to be lowered. The steering stability is likely to improve by increasing a thickness of the conductive portion 18, however, in this case, an improving effect by using the non-conductive rubber for the tread rubber, that is, an effect of reducing a rolling resistance and an effect of improving wet braking performance will be sacrificed.
On the other hand, in the tread rubber 10b in FIG. 6, a contact area of the conductive portion 19 with respect to the non-conductive rubber is large as compared with the tread rubber 10a, and it is comparatively easy to secure the steering stability. However, since a thickness of the conductive portion 19 is uniform, the non-conductive rubber cannot be sufficiently supported and the steering stability tends to be lowered if the thickness is small, and this is disadvantageous in view of forming efficiency of the tread rubber. Further, if the thickness of the conductive portion 19 is large, the improving effect by using the non-conductive rubber for the tread rubber will be sacrificed.
In the tread rubber 10c in FIG. 7, since the thickness of the conductive portion 20 becomes large in a bottom surface side of the tread rubber 10c, and a volume of a base portion greatly contributing to the reduction of the rolling resistance is reduced, the effect of reducing the rolling resistance tends to be deteriorated. Further, a support from a lateral direction by the conductive portion 20 is not sufficient with respect to the non-conductive rubber at the time of cornering, and steering stability tends to be lowered.
In the tread rubber 10d in FIG. 8, a contact area of the conductive portion 21 with respect to the non-conductive rubber is large as compared with the tread rubber 10a, and it is comparatively easy to secure the steering stability. However, since the thickness of the conductive portion 21 is large in the bottom surface side of the tread rubber 10d and the volume of the base portion is reduced, the effect of reducing the rolling resistance tends to be deteriorated. In addition, since the thickness of the conductive portion 21 is large in the ground-contacting surface side, and the non-conductive rubber in contact with the road surface is reduced, the effect of improving the wet braking performance tends to be deteriorated.